Method for steering a towed acoustic linear antenna

ABSTRACT

A method and apparatus are provided for steering a first acoustic linear antenna belonging to a plurality of acoustic linear antennas towed by a vessel. A plurality of navigation control devices are arranged along the plurality of linear antennas in order to act at least laterally on the position of the linear antennas. At least one of the navigation control devices arranged along the first acoustic linear antenna performs steps of: obtaining a local measurement of a feather angle or of a parameter linked to the feather angle, the local measurement being associated with the at least one of the navigation control devices arranged along the first acoustic linear antenna; computing a lateral force, as a function of the obtained local measurement; and applying the computed lateral force.

1. FIELD OF THE DISCLOSURE

The field of the disclosure is the acquisition of geophysics data. Itdeals with the equipments required in order to study the sea bed and itssediment layers properties.

More specifically, the disclosure pertains to a technique for steering atowed acoustic linear antenna.

The disclosure can be applied notably to the oil prospecting industryusing seismic method (sea oil survey), but can be of interest for anyother field which requires a system performing geophysics dataacquisition in a marine environment.

2. TECHNOLOGICAL BACKGROUND

It is sought more particularly here below in this document to describeproblems existing in the field of seismic data acquisition for oilprospecting industry. The present disclosure of course is not limited tothis particular field of application but is of interest for anytechnique that has to cope with closely related or similar issues andproblems.

The operations of acquiring seismic data on site conventionally usenetworks of sensors (here below designated as “hydrophones” with regardto the acquisition of data in a marine environment). Arrays ofhydrophones are forming channels. Several channels are distributed alongcable in order to form linear acoustic antennas normally referred to as“streamers” or “seismic streamers”.

As shown in FIG. 1, the network of seismic streamers 20 a to 20 e istowed by a seismic vessel 21. The hydrophones are referenced 16 in FIG.2, which illustrates in detail the block referenced C in FIG. 1 (i.e. aportion of the streamer referenced 20 a).

The seismic method is based on analysis of reflected seismic waves.Thus, to collect geophysical data in a marine environment, one or moresubmerged seismic sources are activated in order to propagateomni-directional seismic wave trains. The pressure wave generated by theseismic source passes through the column of water and insonifies thedifferent layers of the sea bed. Part of the seismic waves (i.e.acoustic signals) reflected are then detected by the hydrophonesdistributed over the length of the seismic streamers. These acousticsignals are processed and retransmitted by telemetry from the seismicstreamers to the operator station situated on the seismic vessel, wherethe processing of the raw data is carried out.

In practice, it is aimed to carry out an analyze of sea bed with aminimum number of passage of the vessel in the concerned area. For thatpurpose, the number of streamers implemented in the acoustic network issubstantially raised and the length of the streamers may vary between 6and 15 kilometers, for example.

Control of the positions of streamers lies in the implementation ofnavigation control devices, commonly referred to as “birds” (whitesquares referenced 10 in FIG. 1). They are installed at regularintervals (every 300 meters for example) along the seismic streamers.The function of those birds is to guide the streamers betweenthemselves. In other words, the birds are used to control the depth aswell as the lateral position of the streamers. For this purpose, and asillustrated in FIG. 2, each bird 10 comprises a body 11 equipped withmotorized pivoting wings 12 (or more generally means of mechanicalmoving) making it possible to modify the position of the streamerslaterally between them (this is referred to a horizontal driving) anddrive the streamers in immersion (this is referred to a verticaldriving).

To carry out the localization of the seismic streamers (allowing aprecise horizontal driving of the streamers by the birds), acousticnodes are distributed along the streamers. These acoustic nodes arerepresented by hatched squares, referenced 14, in FIGS. 1 and 2. Asshown in FIG. 1, some acoustic nodes 14 of the network are integrated ina bird 10 (case of FIG. 2), and other are not.

The acoustic nodes 14 use underwater acoustic communication means,hereafter referred to as electro-acoustic transducers, allowing toestimate the distances between acoustic nodes (named here below“inter-node distances”). More specifically, these transducers aretransmitters and receivers of acoustic signals, which can be used toestimate an inter-node distance separating two acoustic nodes (acting assender node and receiver node respectively) situated on two differentstreamers (which may be adjacent or not) as a function of an acousticsignal propagation duration measured between these two nodes (i.e. atravel time of the acoustic signal from the sender node to the receivernode). From the acoustic network, this thereby forms a mesh ofinter-node distances allowing to know precise horizontal steering of allthe streamers. Transducer here is understood to mean either a singleelectro-acoustic device consisting of a transceiver (emitter/receiver)of acoustic signals, or a combination of a sender device (e.g. a pinger)and a receiver device (e.g. a pressure particle sensor (hydrophone) or amotion particle sensor (accelerometer, geophone . . . )). Usually, eachacoustic node comprises an electro-acoustic transducer enabling it tobehave alternately as a sender node and a receiver node (for thetransmission and the reception, respectively, of acoustic signals). Inan alternative embodiment, a first set of nodes act only as sender nodesand a second set of nodes act only as receiver nodes. A third set ofnodes (each acting alternately as a sender node and a receiver node) canalso be used in combination with the first and second sets of nodes.

The inter-node distance d_(AB) between two nodes A and B can betypically estimated on the basis of the following formula:d_(AB)=c·t_(AB), with: node A acting as a sender node which transmits anacoustic signal S to node B acting as a receiver node (see example inFIG. 1, with acoustic signal S shown as an arrow between nodesreferenced A and B); t_(AB), the propagation duration (travel time)elapsed between the emission instant and reception instant of theacoustic signal transmitted from the sender node A to the receiver nodeB (assuming that the receiver node and the sender node aresynchronized); and c, a “measured” or “estimated” value of sound speed(also referred to as sound velocity) of the acoustic signal.

FIG. 3 illustrates the binning coverage. We consider two successiveshots of a seismic source: the first shot is illustrated in the upperpart of FIG. 3 and the second shot is illustrated in the lower part ofFIG. 3 (i.e. the source and the streamer are towed from the right to theleft in this example). At each shot of the seismic source, a step ofprocessing is the assignment of each channel to a bin. Bins representlocal areas (e.g. 8 m×8 m) on the Earth's surface which have been probedby some channels during the seismic survey, i.e. which have been hit bysome rays coming from the source and whose reflected ray is received bya channel.

When several traces can be assigned to the same bin, then the signal tonoise ratio may be improved with a processing called “stacking” of theseismic data. The number of different rays reflected on the same bin iscalled “coverage”. One of the aims of a seismic survey is to get auniform coverage of the binning grid. However, different events canaffect the coverage of the binning grid, such as a feather angle α onthe streamers S1-S4 (towed by a seismic vessel 21 via a head rigging 43)caused by a lateral sea current 41 (as illustrated in FIG. 4), a V-shapeof the streamer network caused by the vessel's wash, or more generallythe distortion of streamers. The feather angle α is the angle formed bya streamer (e.g. S1) relative to the axis 42 along which the vessel 21moves.

During seismic surveys, the areas to cover are actually skimmed bylines. If we observe a binning grid, with the coverage of each bin, wecan see some gaps between adjacent lines which are mainly due to featherangle effect on the network. When the coverage between adjacent lines ofthe survey is poor, then additional lines called “infill lines” arerequired, which is time and cost-consuming.

In the last decade, prospectors have equipped the streamers withinstruments which permit to control them laterally. As already discussedabove, these instruments are navigation control devices (“birds”) whichallow maintaining a lateral distance between streamers, which have theeffect of suppressing the V-shape and any individual streamerdistortion. Sometimes, these instruments are also used to guarantee astable V-shape, which is also beneficial for coverage.

Besides, some current models which include meteorological data andsatellite observations, added by onboard Acoustic Doppler CurrentProfiling (ADCP) permit predicting streamer distortion and controllingthe navigation control devices (“birds”) as a function of the currentprediction information. This allows to minimize “infill lines” and tomaximize four dimensional (4D) repeatability. A four dimensional seismicsurvey is a three dimensional survey over a same area of the Earth'ssubsurface at selected time.

However, despite the integration of navigation control devices (“birds”,i.e. means of lateral control of streamers), there is still sometimes ahad coverage of the seismic area and/or a lack of repeatability, mainlydue to the feather angle of streamers which can change during a vintageand from a vintage to another, or between two adjacent lines, creatinggaps in the coverage.

Moreover, on some systems, all the lateral control is referred to areference streamer, also called “master streamer”. In this case, as alladjacent streamers are referred to the master streamer thanks to a localcontrol of the lateral forces, a feather angle of the master streamertends to create the same feather angle for all the spread (i.e. all theadjacent streamers). For example, in FIG. 4, if S is the Masterstreamer, the slave streamers S2, S3 and S4 have the same feather angleα as S1.

Another drawback of these systems is that if no global control of thenavigation control devices (in order to operate a lateral control of themaster streamer) is carried out, the shape and direction of the masterstreamer vary with the current, inducing a feather angle on thestreamers caused in case of lateral sea current. If a global control iscarried out by a navigation system (on board of the seismic vessel),this is not an optimal solution to keep a stable network in thefollowing situations:

-   -   disconnection or cut of streamers, each streamer being connected        to a seismic data acquisition system onboard the vessel;    -   break on a telemetry line between a navigation control device        (“bird”) and the onboard control system of the navigation        control devices;    -   loss of the link between the navigation system and the control        system of the navigation control devices.

It must also be noted that between each line of a seismic survey, thevessel realizes a turn of approximately 3°/mn. The time required tostabilize the streamer network is important and is mainly dependant onthe vessel speed because the network is generally free in feather angle.The streamer network distortion due to a turn is close to the streamernetwork distortion doe to a lateral sea current. Therefore, thedifferent drawbacks of the prior art solutions, described above in thecase of a lateral sea current are substantially the same the case of aturn.

3. SUMMARY

A particular embodiment of the invention proposes a method for steeringa first acoustic linear antenna belonging to a plurality of acousticlinear antennas towed by a vessel, a plurality of navigation controldevices being arranged along said plurality of linear antennas in orderto act at least laterally on the position of said linear antennas. Atleast one of the navigation control devices arranged along said firstacoustic linear antenna performs steps of:

-   -   obtaining a local measurement of a feather angle or of a        parameter linked to the feather angle, said local measurement        being associated with said at least one of the navigation        control devices arranged along said first acoustic linear        antenna;    -   computing a lateral force, as a function of the obtained local        measurement; and    -   applying the computed lateral force.

Thus, this particular embodiment relies on a wholly novel and inventiveapproach taking advantage of the fact that the feather angle (or aparameter linked to the feather angle) is used as an input informationto compute the lateral force to apply. This allows to improve binningcoverage and 4D repeatability of marine surveys.

According to a particular feature, each of the navigation controldevices arranged along said first acoustic linear antenna performs saidsteps of obtaining, computing and applying.

Thus, the steering is optimized.

According to a particular feature, said first linear antenna is a masterlinear antenna to which at least one slave linear antenna, belonging tosaid plurality of acoustic linear antennas, is referred to.

Thus, an embodiment the invention is carried out only once, for themaster linear antenna (master streamer).

According to a particular feature, said parameter linked to the featherangle is a distance separating:

-   -   a reference acoustic node, arranged along another linear antenna        among said plurality of acoustic linear antennas; and    -   a point which is an orthogonal projection, on said other        acoustic linear antenna, of said at least one of the navigation        control devices.

According to a particular feature, the step of computing the lateralforce comprises a step of carrying out a control loop which enslaves theobtained local measurement on a setpoint.

Thus, the computing of the lateral force is implemented in a simplemanner.

In a first implementation, said setpoint is a predetermined value.

This allows to keep an optimised feather angle of the linear antenna(streamer), even if none connection to a master streamer controller isavailable (streamer disconnection, a master streamer controller poweredoff, etc).

In a second implementation, the step of computing the lateral forcecomprises a step of dynamically computing said setpoint, as a value of aglobal feather angle or a global parameter linked to the global featherangle, said global feather angle being a mean feather angle over saidfirst linear antenna.

Indeed, it may be beneficial for binning coverage or 4D repeatability toremove streamer distortion instead of trying to reduce to apredetermined value (e.g. 7°) the streamer feather angle withoutreducing the streamer distortion.

According to a particular feature of this second implementation, theglobal feather angle is computed as a function of acoustic signals,geodetic positions and compass bearing.

Thus, the global feather angle is computed dynamically.

According to a particular feature, the step of obtaining the localmeasurement comprises steps of:

-   -   obtaining acoustic signals transmitted between acoustic nodes        arranged along a couple of linear antennas comprising said first        linear antenna and another linear antenna among said plurality        of acoustic linear antennas, said acoustic signals being        intended to be used by said acoustic nodes to estimate the        distances between said acoustic nodes;    -   computing the local measurement as a function of the obtained        acoustic signals.

Thus, the method does not require any specific measure equipment (astuteuse of the acoustic nodes, for a new function, in addition to theirprimary function).

According to a particular feature, said other linear antenna is adjacentto said first linear antenna.

Thus, in the case the first linear antenna is a master linear antenna,and the other linear antenna is a slave linear antenna, the hypothesisthat the feather angles of these two linear antennas are equal is betterverified. In other words, there is less bias (error) resulting from thefact feather angles of the two linear antennas (master and slavestreamers) are not perfectly identical.

In a particular implementation, the step of obtaining the localmeasurement comprises steps of:

-   -   obtaining a predetermined distance d_(XY) separating a couple of        first and second acoustic nodes X, Y placed along one of said        couple of linear antennas;    -   obtaining a first propagation duration t_(XD) of an acoustic        signal transmitted between the first acoustic node X and a third        acoustic node D placed along the other of said couple of linear        antennas;    -   obtaining a second propagation duration t_(YD) of an acoustic        signal transmitted between the second acoustic node Y and the        third acoustic node D;    -   obtaining a value k of the underwater acoustic sound velocity;    -   estimating, as a function of t_(XD), t_(YD), k and d_(XY), a        cross-line distance d_(HD) between said first linear antenna and        said other linear antenna, defined as the length of the        altitude, passing through the third node D, of a triangle having        as vertexes the first, second and third nodes X, Y and D, H        being the foot of said altitude;    -   estimating, as a function of t_(XD), k and d_(HD), a distance        d_(XH) or d_(YH) separating the foot H and the first acoustic        node X or the second acoustic node Y;    -   estimating, as a function of d_(XH) and a predetermined distance        d_(XB) separating the first acoustic node X and a fourth        acoustic node B or as a function of d_(YH) and a predetermined        distance d_(YB) separating the second acoustic node Y and the        fourth acoustic node B, a distance d_(HB) separating the foot H        and the fourth acoustic node B, said distance d_(HB) being used        as the parameter linked to the feather angle, said fourth        acoustic node B being arranged along the same linear antenna as        the first and second nodes X, Y;    -   if the obtained local measurement is the local measurement of        the feather angle, estimating the feather angle as a function of        d_(HB) and d_(HD).

According to a particular feature, the fourth acoustic node B iscoincident with the first acoustic node X or the second acoustic node Y.

This allows to simplify the step of obtaining the local measurement.

According to a particular feature, one of said first, second, third andfourth acoustic nodes X, Y, D and B is integrated in said at least oneof said navigation control devices.

This allows also to simplify the step of obtaining the localmeasurement.

According to a particular feature, the step of computing a lateral forceis carried out also as a function of at least one other obtained localmeasurement associated with another one of said navigation controldevices.

This allows to optimize the step of computing the lateral force (e.g.accelerate the enslavement of a control loop).

In another embodiment, the invention pertains to a computer programproduct comprising program code instructions for implementing theabove-mentioned method (in any of its different embodiments) when saidprogram is executed on a computer or a processor.

In another embodiment, the invention pertains to a non-transitorycomputer-readable carrier medium, storing a program which, when executedby a computer or a processor causes the computer or the processor tocarry out the above-mentioned method (in any of its differentembodiments).

In another embodiment, the invention proposes a navigation controldevice arranged along a towed acoustic linear antenna in order to act atleast laterally on the position of said linear antenna, characterized inthat it comprises:

-   -   means for obtaining a local measurement of a feather angle or of        a parameter linked to the feather angle, said local measurement        being associated with said navigation control device;    -   means for computing a lateral force, as a function of the        obtained local measurement; and    -   means for applying the computed lateral force.

4. LIST OF FIGURES

Other features and advantages of embodiments of the invention shallappear from the following description, given by way of an indicative andnon-exhaustive examples and from the appended drawings, of which:

FIG. 1, already described with reference to the prior art, presents anexample of network of seismic streamers towed by a seismic vessel;

FIG. 2, already described with reference to the prior art, illustratesin detail the block referenced C in FIG. 1 (i.e. a portion of astreamer);

FIG. 3, already described with reference to the prior art, illustratesthe binning coverage;

FIG. 4, already described with reference to the prior art, illustrates afeather angle on streamers (towed by a seismic vessel) caused by alateral sea current;

FIGS. 5A and 5B illustrate the general principle of the method accordingto an embodiment the invention, implemented in a navigation controldevice (bird), in the particular case when a reference acoustic node Bis coincident with a point H (defined below) when the local featherangle is a zero angle;

FIG. 6 illustrates a global feather angle α_(g), used in an alternativeembodiment of the invention;

FIG. 7 shows the simplified structure of a navigation control device(bird) according to a particular embodiment of the invention;

FIG. 8 is a flowchart of a particular embodiment of the method accordingto the invention;

FIGS. 9A and 9B illustrate an alternative case (compared with theparticular case of FIGS. 5A and 5B) when the reference acoustic node Bis not coincident with the point H when the local feather angle is azero angle.

5. DETAILED DESCRIPTION

In all of the figures of the present document, identical elements andsteps are designated by the same numerical reference sign.

The method described below, in the case of a lateral sea current, canalso be applied in the case of a turn, in order to allow stabilizingquickly the streamer network and therefore decreasing the time spent inturns.

In the illustrative embodiment shown in FIGS. 5A and 5B, we consider astreamer network comprising a master streamer 51 and a slave streamer52, towed by a vessel (not shown) via a head rigging 53.

It is clear however that an embodiment of the invention can beimplemented with a streamer network comprising a greater number of slavestreamers, and/or with more than one master streamer (in this case, themethod according to the invention is carried out for each masterstreamer). More generally, an embodiment of the invention can be appliedto control lateral steering of any streamer.

As already explained above in relation with FIGS. 1 and 2, navigationcontrol devices (“birds”) and acoustic nodes are arranged along eachstreamer 51, 52. Some acoustic nodes are integrated in a bird (case ofFIG. 2), and other are not.

In the example illustrated in FIGS. 5A and 5B, we consider threeacoustic nodes A, B and C arranged along the slave streamer 52, and onebird D, in which is integrated an acoustic node (called acoustic node Dthereafter), arranged along the master streamer 51.

We discuss now a method according to a particular embodiment of theinvention, implemented in the bird D. In practice, this method can beimplemented in all (or almost all) the birds arranged along the masterstreamer 51 (or along each of the master streamers when there areseveral).

As already defined above, the feather angle α of a streamer is definedas the angle formed by this streamer relative to the axis along whichthe vessel moves. Considering that the slave streamer 52 and the masterstreamer 51 are parallels, then the feather angle α is the same for eachstreamer.

In FIG. 5A, we assume that the streamers 51, 52 are parallels to theaxis 54 along which the vessel moves, then the feather angle α is equalto zero. In FIG. 5B, we assume that the streamers 51, 52 are notparallels to the axis 54 along which the vessel moves, theft the featherangle α is different from zero.

The constraint that the acoustic node D must be met in any triangle(XDY) having as vertexes two acoustic nodes X and Y (e.g. any of thefollowing couples: A and C, A and B, B and C) is that said two acousticnodes X and Y must be arranged along the slave streamer 52, and theacoustic node D must be arranged along the master streamer 51.

Computation of the Cross-Line Distance d_(HD)

A cross-line distance d_(HD) between the slave and master linearantennas 51, 52 is defined as the length of the aforesaid altitude(having the foot H and passing through the acoustic node D, in thetriangle (XDY).

We explain now how the cross-line distance d_(HD) can be computed if thelength of the sides of the triangle (XDY) are known. We note the lengthsof the sides as follows: d_(XY)=c, d_(YD)=a, d_(AD)=b and h the lengthof the altitude HD. By Heron's formula, the area of this triangle is:Aera_(XDY)=√{square root over (s(s−a)(s−b)(s−c))}where s=½(a+b+c) is half of the triangle's perimeter.

But the area of a triangle can also be written with the well-knownformula:

${Aera}_{XDY} = \frac{c \cdot h}{2}$where h (also noted d_(HD)) is the altitude having the foot H andpassing through the vertex D, and c is the length of the base XY of thetriangle XDY.

From these two above formulas for calculating area of the triangle XDY,we obtain the following:

${{s( {s - a} )}( {s - b} )( {s - c} )} = \frac{c^{2}h^{2}}{4}$which, after simplifying, leads to the following formula (I):

$\begin{matrix}{d_{HD}^{2} = {h^{2} = {- \frac{( {a + b + c} )( {a + b - c} )( {a - b - c} )( {a - b + c} )}{4\; c^{2}}}}} & (I)\end{matrix}$

In the example of FIGS. 5A and 5B, the cross-line distance d_(HD) can becomputed in each of the following cases, since triangles (ADB), (ADC)and (BDC) have the same altitude HD:

-   -   if the length of the sides of the triangle (ADB) are known,    -   if the length of the sides of the triangle (ADC) are known,    -   if the length of the sides of the triangle (BDC) are known.

If several values of the cross-line distance d_(HD) are computed (eachin a different triangle), then a final value of the cross-line distanced_(HD) is equal to the mean of these values (or any other combination ofthese values).

Computation of the Distance d_(HB)

We consider a distance d_(HB) separating the foot H (which can bedefined also as the orthogonal projection of the node D on the slavestreamer 52) and a reference acoustic node B (arranged along the slavestreamer 52).

In the particular embodiment of FIGS. 5A and 5B, the reference acousticnode B is coincident with the foot H when the local feather angle is azero angle (α=0). In other words, the distance d_(HB) is equal to zerowhen α is equal to zero, i.e. in the case of FIG. 5A (since B=H), and isdifferent from zero when α is different from zero, i.e. in the case ofFIG. 5B. In this particular embodiment, the distance d_(HB) is theinline offset.

In alternative embodiments (e.g. in FIGS. 9A and 9B, in which thefeather angle α is equal to zero and different from zero respectively),the reference acoustic node B is not coincident with the foot H when thelocal feather angle is a zero angle (α=0, see FIG. 9A). Therefore, whenα=0 (as shown in FIG. 9A), the distance d_(HB) is not equal to zero butto H₀B, with H₀ the orthogonal projection of the node D when α=0. Inthese alternative embodiments, the distance d_(HB) is not the inlineoffset, the inline offset being the distance HH₀.

We explain now how the distance d_(HB) can be computed with Pythagoras'stheorem, if the cross-line distance d_(HD) and some inter-node distancesare known or computed (using acoustic signals corning from the adjacentstreamer, and considering that the underwater acoustic sound velocity isknown).

In a first example, we consider the triangle (ADB) to compute thecross-line distance d_(HD), according to above formula (I), withd_(AB)=c, d_(BD)=a, d_(AD)=b. The distance d_(AB) (between the acousticnodes A and B) is known. The distances d_(BD) (between the acousticnodes B and D) and d_(AD) (between the acoustic nodes A and D) arecomputed according to the following formula: d_(BD)=t_(BD)×k andd_(AD)=t_(AD)×k, with t_(BD) the acoustic propagation time betweenacoustic nodes B and D, and t_(AD) the acoustic propagation time betweenacoustic nodes A and D. t_(BD) and t_(AD) are measured by the acousticnode D. k is the underwater acoustic sound velocity (which is eithermeasured or estimated).

Then we have: d_(AH)=√{square root over (d_(HD) ²−d_(HD) ²)}

And finally: d_(HB)=d_(AB)−d_(AH)

Per convention, d_(HB)>0 if H is between A and B, and d_(HB)<0otherwise. In this first example, H is not between A and B.

In a second example, we consider the triangle (BDC) to compute thecross-line distance d_(HD), according to above formula (I), withd_(CB)=c, d_(BD)=a, d_(CD)=b. The distance d_(CB) (between the acousticnodes C and B) is known. The distances d_(BD) (between the acousticnodes B and D) and d_(CD) (between the acoustic nodes C and D) arecomputed according to the following formula: d_(BD)=t_(BD)×k andd_(CD)=t_(CD)×k, with t_(BD) the acoustic propagation time betweenacoustic nodes B and D, and t_(CD) the acoustic propagation time betweenacoustic nodes C and D. t_(BD) and t_(CD) are measured by the acousticnode k is the underwater acoustic sound velocity (which is eithermeasured or estimated).

Then we have: d_(CH)=√{square root over (d_(CD) ²−d_(HD) ²)}

And finally: d_(HB)=d_(CB)−d_(CH)

Per convention, d_(HB)>0 if H is between B and C, and d_(HB)<0otherwise. In this second example, H is between B and C.

In a third example, we consider the triangle (ADC) to compute thecross-line distance d_(HD), according to above formula (I), withd_(AC)=c, d_(AD)=b, d_(CD)=a. The distance d_(AC) (between the acousticnodes A and C) is known. The distances d_(AD) (between the acousticnodes A and D) and d_(CD) (between the acoustic nodes C and D) arecomputed according to the following formula: t_(AD)=t_(AD)×k andd_(CD)=t_(CD)×k, with t_(AD) the acoustic propagation time betweenacoustic nodes A and D, and t_(CD) the acoustic propagation time betweenacoustic nodes C and D. t_(AD) and t_(CD) are measured by the acousticnode D. k is the underwater acoustic sound velocity (which is eithermeasured or estimated).

Then we have: d_(CH)=√{square root over (d_(CD) ²−d_(HD) ²)}

And finally: d_(HB)=d_(CB)−d_(CH), where d_(CB) (the distance betweenthe acoustic nodes C and B) is known.

Per convention, d_(HB)>0 if H is between B and C, and d_(HB)<0otherwise. In this third example, H is between B and C.

Computation of the Feather Angle α

The distance d_(HB) is a parameter linked to the feather angle α,because of the head streamer rigging 53 which maintains a same offset ofthe streamers per comparison to the vessel.

In the particular embodiment of FIGS. 5A and 5B, knowing the distanced_(HB) (which is also the inline offset in this particular embodiment)and the cross-line distance d_(HD), the feather angle α can be computedusing the following formula (II):

$\begin{matrix}{\alpha = {\arctan( \frac{d_{HB}}{d_{HD}} )}} & ({II})\end{matrix}$

Computation of the Lateral Force

The bird D computes the lateral force to be applied (by the motorizedpivoting wings 12), as a function of the computed distance d_(HB) (localmeasurement of a parameter linked to the feather angle α).

In a particular embodiment, the lateral force is computed with a localPID (“Proportional-Integral-Derivative”) control loop which enslaves thecomputed distance d_(HB) (also referred to as “computed inline offset”in the particular embodiment of FIGS. 5A and 5B) on a setpoint.

As detailed below, in relation with FIG. 8, the setpoint is either apredetermined value or a dynamically computed value.

In an alternative embodiment, the bird D computes the lateral force as afunction of the computed feather angle α (local measurement of thefeather angle α), e.g. with a local PID control loop which enslaves thecomputed feather angle α on a feather angle setpoint.

In an alternative embodiment, the computation of the distance d_(HB) (orof the feather angle α) is carried out by another device D′; e.g. anacoustic node which is close to the bird D (D must be replaced by D′ inall above computation details and formulas). Then the bird D receivesthe measurement (i.e. the computed distance d_(HB) or the computedfeather angle α) and computes the lateral force as a function of thisreceived measurement and a setpoint.

In an alternative embodiment, the computation of the lateral force isalso carried out by aforesaid other device. Then the bird D receivesonly the computed lateral force (i.e. a signal indicating to the bird Dwhich lateral force has to be applied).

The left part of FIG. 8 is a flowchart of a particular embodiment of themethod according to the invention, implemented by the bird D (see FIGS.5A and 5B).

In a step 81, the bird D obtains acoustic propagation times (e.g.t_(AD), t_(BD) and t_(CD)), known inter-node distances (e.g. d_(AB),d_(BC) and d_(AC)) and the underwater acoustic sound velocity (k, whichis either measured or estimated).

In a step 82, the bird D computes the distance d_(HB) (local measurementof a parameter linked to the feather angle α).

In a step 83, the bird D computes a lateral force LT to be applied, as afunction of the computed distance d_(HB), e.g. with a local PID controlloop which enslaves the computed distance d_(HB) on a setpoint SP.

In a step 84, the bird D applies (with the motorized pivoting wings 12)the computed lateral force LF.

First Implementation

In a first implementation, the setpoint SP is a predetermined value (forexample corresponding to a feather angle of 0°, 5° or 10°). Thispredetermined value can be stored in a memory of the bird D. It can alsobe sent to the bird D by a master streamer controller (comprised e.g. inthe navigation system or any other control system, on board of theseismic vessel).

In order to locally compute the lateral force to apply, the bird D onlytakes in account the locally measured distance d_(HB) and thepre-established set point.

This allows to keep an optimised feather angle of the master streamer51, even if none connection to the master streamer controller isavailable (streamer disconnection, a master streamer controller poweredoff, etc).

As the slave streamers (52 in FIGS. 5A and 5B) keep a separation withthe attached master streamer, then the feather angle is reduced on thewhole streamer network, even if none connection is available between themaster streamer 51 and the master streamer controller.

Second Implementation

In practice, the streamer feather angle α caused by the lateral seacurrent can regularly be much higher than 10°, whereas the birds cantypically correct a feather angle of approximately 3°.

Moreover, the distance d_(HB) (i.e. the inline offset in the particularembodiment of FIGS. 5A and 5B) measured by each bird along a streamermay be different due to the streamer distortion.

Each function, streamer angle reduction and distortion reduction, has acost on the lateral force which can be expressed in lateral forcedynamic. Then if, on a master streamer the whole dynamic of lateralforce is used for streamer angle reduction, then the streamer still maybe distorted.

Moreover, in that case the whole dynamic of lateral force of the slavestreamer may also be used, as the slave streamer try to keep a fixedstreamer separation (cross-line distance) with respect to the attachedmaster streamer. It could result to a loss of streamer separation, whichcan be worth than a presence of a feather angle.

Finally, it may be beneficial for binning coverage or 4D repeatabilityto remove streamer distortion instead of trying to reduce to 7° thestreamer feather angle without reducing the streamer distortion.

So, in a second implementation, the method according to the invention isused to keep a same global feather angle α_(g) along a streamer,reducing the streamer distortion, and keeping a free dynamic of lateralforce on the birds of slave streamers.

As illustrated in FIG. 6, the global feather angle α_(g) (also referredto as the “streamer feather angle”), is the mean feather angle over astreamer (different of the feather angle measured by a bird, alsoreferred to above as “the local measurement of the feather angle”). Itcan be computed in real time by the navigation system (step 92, partright of FIG. 8), e.g. with a least square algorithm which uses, asinputs, acoustic signals, geodetic positions and compass bearing (theseinputs are obtained in step 91, part right of FIG. 8).

The streamer feather angle α_(g) can be used to determine the set pointSP for all the birds along the master streamer 51 (step 93, part rightof FIG. 8). Then the distortion along the streamers may be reducedwithout modifying the streamer feather angle.

In that case, and in the particular context of FIGS. 5A and 5B, the setpoint SP is defined by:SP=CROSSLINE_DISTANCE_SETPOINT·tan(α_(g))where CROSSLINE_DISTANCE_SETPOINT is the streamer separation (crosslinedistance) setpoint, and α_(g) is the effective streamer feather angle.

Now referring to FIG. 7, we present the simplified structure of anavigation control device 70 (bird D in the above discussion) accordingto a particular embodiment of the invention.

The navigation control device 70 comprises a read-only memory (ROM) 73,a random access memory (RAM) 71 and a processor 72. The read-only memory73 (non transitory computer-readable carrier medium) stores executableprogram code instructions, which are executed by the processor 72 inorder to enable implementation of the technique of an embodiment of theinvention (e.g. the steps 81 to 84 of FIG. 8).

Upon initialization, the aforementioned program code instructions aretransferred from the read-only memory 73 to the random access memory 71so as to be executed by the processor 72. The random access memory 71likewise includes registers for storing the variables and parametersrequired for this execution. The processor 72 receives the followinginformation:

-   -   acoustic propagation times (e.g. t_(AD), t_(BD) and t_(CD));    -   known inter-node distances (e.g. d_(AB), d_(BC) and d_(AC));    -   a measured value of the underwater acoustic sound velocity (k).        In an alternative embodiment, this value is estimated by the        processor 72 (see Patent Application EP 11305835.8);    -   the setpoint SP.

According to the program code instructions, the processor 72 deliversthe computed lateral force LF (see steps 82 and 83 in FIG. 8), to beapplied by the motorized pivoting wings 12.

All the steps of the above estimation method can be implemented equallywell:

-   -   by the execution of a set of program code instructions executed        by a reprogrammable computing machine such as a PC type        apparatus, a DSP (digital signal processor) or a        microcontroller. This program code instructions can be stored in        a non-transitory computer-readable carrier medium that is        detachable (for example a floppy disk, a CD-ROM or a DVD-ROM) or        non-detachable; or    -   by a dedicated machine or component, such as an FPGA (Field        Programmable Gate Array), an ASIC (Application-Specific        Integrated Circuit) or any dedicated hardware component.

At least one embodiment of the present disclosure provides a techniquefor steering laterally a towed acoustic linear antenna, this techniqueallowing to improve binning coverage and 4D repeatability of marinesurveys.

At least one embodiment provides a technique of this kind which does notneed any global control of the navigation control devices (“birds”), inorder to keep a stable streamer network, even in the three aforesaidsituations.

At least one embodiment provides a technique of this kind which allowsto decrease the time spent in turns, by speeding up the stabilisation ofthe streamer network at the end of a turn.

At least one embodiment provides a technique of this kind that is simpleto implement and costs little.

The invention claimed is:
 1. A method for steering in water a masteracoustic linear antenna, the method comprising: receiving acousticpropagation times between (i) first, second and third nodes located on aslave acoustic linear antenna and (ii) a node located on the masteracoustic linear antenna, wherein the slave linear acoustic antennafollows a shape of the master linear acoustic antenna; calculating, inat least one navigation control device attached to the node located onthe master acoustic linear antenna, a local measurement of (i) a featherangle or (ii) a parameter linked to the feather angle based on theacoustic propagation times, inter-node distances between the first,second and third nodes along the slave acoustic linear antenna, and aspeed of sound in water; obtaining a feather angle set point; applyingwith the at least one navigation control device a lateral force to themaster acoustic linear antenna; and controlling, based on the localmeasurement and the feather angle set point, the applied lateral force.2. The method of claim 1, further wherein the applying of the lateralforce is achieved with motorized pivoting wings of the at least onenavigation control device.
 3. The method of claim 1, wherein the atleast one navigation control device is a bird that controls a lateralmovement of the master acoustic linear antenna.
 4. The method of claim1, further comprising: performing a control loop with the at least onenavigation control device for controlling the lateral force.
 5. Themethod of claim 4, wherein the control loop is implemented with aproportional-integral-derivative loop.
 6. The method of claim 1, whereinthe at least one navigation control device includes a processor thatreceives as input (1) the acoustic propagation times between thenavigation control device and the first, second and third nodes, (2) theinter-node distances, (3) the speed of sound in water, and (4) thefeather angle set point, and the processor controls the lateral forceapplied by motorized pivoting wings based on inputs (1) to (4).
 7. Themethod of claim 1, wherein the feather angle set point has apredetermined value.
 8. The method of claim 7, wherein the predeterminedvalue is sent by a master streamer controller.
 9. The method of claim 1,wherein the feather angle set point is dynamically computed.
 10. Themethod of claim 1, wherein the feather angle set point is a globalfeather angle of the master acoustic linear antenna.
 11. The method ofclaim 10, wherein the local measurement is the feather angle and thelocal measurement of the feather angle characterizes a part of themaster acoustic linear antenna.
 12. The method of claim 1, wherein thestep of calculating comprises: selecting the at least one navigationcontrol device (D) attached to the master acoustic linear antenna and areference node (B) attached to the slave acoustic linear antenna;calculating a cross-line distance d_(HD) between the master and slaveacoustic linear antennas, wherein point H is on the slave acousticlinear antenna and the distance d_(HD) is perpendicular to both themaster and slave acoustic linear antennas; and calculating a distanced_(HB) between the reference node (B) and point H, wherein the distanced_(HB) is the parameter linked to the feather angle which is the localmeasurement.
 13. A navigation control device for steering in water amaster acoustic linear antenna, the navigation control devicecomprising: a processor that receives acoustic propagation times between(i) first, second and third nodes located on a slave acoustic linearantenna and (ii) a node located on the master acoustic linear antenna,wherein the slave linear acoustic antenna follows a shape of the masterlinear acoustic antenna, calculates a local measurement of (i) a featherangle or (ii) a parameter linked to the feather angle of the masteracoustic linear antenna based on the acoustic propagation times,inter-node distances between the first, second and third nodes along theslave acoustic linear antenna, and a speed of sound in water, andobtains a feather angle set point; and at least one navigation controldevice having motorized wings that apply a lateral force to the masteracoustic linear antenna, wherein the processor controls, based on thelocal measurement and the feather angle set point, the applied lateralforce through the motorized wings.
 14. The navigation control device ofclaim 13, wherein the processor implements aproportional-integral-derivative loop for controlling the lateral force.15. The navigation control device of claim 13, wherein the processorreceives as input (1) acoustic propagation times between the navigationcontrol device and the first, second and third nodes, (2) the inter-nodedistances, (3) the speed of sound in water, and (4) the feather angleset point, and the processor controls the lateral force to be applied bymotorized pivoting wings based on inputs (1) to (4).
 16. The navigationcontrol device of claim 13, wherein the feather angle set point is aglobal feather angle of the master acoustic linear antenna.
 17. Thenavigation control device of claim 16, wherein the local measurement isthe feather angle and the local measurement of the feather anglecharacterizes a part of the master acoustic linear antenna.
 18. Thenavigation control device of claim 17, wherein the slave linear acousticantenna follows a shape of the master linear acoustic antenna.
 19. Anon-transitory computer readable medium storing a program which, whenexecuted by a processor, causes the processor to perform the steps of:receiving acoustic propagation times between (i) first, second and thirdnodes located on a slave acoustic linear antenna and (ii) a node locatedon the master acoustic linear antenna, wherein the slave linear acousticantenna follows a shape of the master linear acoustic antenna;calculating, in at least one navigation control device attached to themaster acoustic linear antenna, a local measurement of (i) a featherangle or (ii) a parameter linked to the feather angle based on theacoustic propagation times, inter-node distances between the first,second and third nodes along the slave acoustic linear antenna, and aspeed of sound in water; obtaining a feather angle set point; applyingwith the at least one navigation control device a lateral force to themaster acoustic linear antenna; and controlling, based on the localmeasurement and the feather angle set point, the applied lateral force.20. The method of claim 1, wherein the lateral force is determined so asto make the feather angle match the set angle and/or to reducedistortion of the master streamer.